There's another new, gaping hole in the middle of the Browns' defense.

They may actually be able to plug it this time.

Starting inside linebacker D'Qwell Jackson, who initially feared he might be lost for the season, will be sidelined for at least a few weeks with a strained right pectoral muscle, the second chest muscle injury the dependable four-year veteran has sustained in less than a year.

On Thursday, Jackson got positive news on the injury when he got a second opinion from the surgeon who reattached his left pectoral muscle in October. Dr. Frank Cordasco agreed with the diagnosis by Cleveland's medical staff and determined Jackson will not need surgery, said a person familiar with the injury who spoke on condition of anonymity because of medical privacy laws.

Jackson will begin rehab and should be back within 3 to 7 weeks, the person said. If he doesn't have any setbacks, Jackson could return for the Sept. 12 opener at Tampa Bay.

When he got hurt on Tuesday, Jackson, who missed Cleveland's final 10 games last season with the chest injury, left the field dejectedly and was visibly upset when he returned to watch the final 30 minutes.

The temporary loss of Jackson will force the Browns to shuffle their linebacking group, a unit the club strengthened during the offseason by signing free agent Scott Fujita, who won a Super Bowl ring with New Orleans, and acquiring Chris Gocong, a three-year player with Philadelphia, in a trade.

Gocong has been playing alongside Jackson in the interior of Cleveland's 3-4 alignment during camp. With Jackson out, Eric Barton likely will slide into the middle, a spot he owned until injuring his neck and sitting out the Browns' final eight games last season.

Linebacker is the one position where the Browns can claim to have some depth. They're about to find out how strong it really is.

"This will be a test," Gocong said following the first of two practices Thursday as the Browns got ready for their exhibition opener Saturday in Green Bay. "We're definitely going to move guys around and see what fits where. We're going to have to move some guys around."

Along with Fujita, Gocong and Barton, the Browns could use David Bowens, Kaluka Maiava and Jason Trusnik in the middle.

"We've got a lot of guys who can do a lot of things," Trusnik said.

Coach Eric Mangini has a comfort level with Barton, who was leading the Browns in tackles when he got hurt last season. Mangini and Barton spent three years together in New York, and the coach trusts his knowledge in coordinator Rob Ryan's attacking system and ability to make the right calls.

Mangini knows nothing can replace hands-on experience.

"It's nice to have the guys that we have with the experience that we have here," Mangini said. "Eric Barton has started over 100 games through the course of his career, and Chris with his experience and David Bowens with this experience, Kaluka (Maiava) got some time last year as a young guy, right on down the line. I think it does help."

On paper, Cleveland's defense looks better than last season's group, which finished ranked 31st overall, 28th against the run. Ryan and Mangini are optimistic that his defense will be vastly improved, but they won't know for certain until the season kicks off.

Meanwhile, the Browns will miss more than Jackson's tackling. One of the team's most respected players, he impressed his teammates with a strong offseason of conditioning. Disappointed that the Browns would not sign him to a long-term contract, Jackson signed a one-year tender worth $1.76 million.

He was counting on a big year to parlay into a big, free agent contract next winter. Now, he faces an uncertain future.

"He's not thinking negative about it," said safety Abram Elam, who spoke with Jackson on Wednesday. "He's trying to bring some positive into it. He's optimistic. Last year when we lost D'Qwell, we had to come together and it gave other guys opportunities. We have to do it again."

Notes: CB Eric Wright was back after missing two days with a leg injury. ... Punter Dave Zastudil has returned to Ohio after visiting his knee surgeon in Colorado. Mangini said there are no immediate plans for Zastudil to have surgery. He had a torn patellar tendon repaired in his right (plant) leg in December after missing eight games in 2009. ... Mangini plans to play QBs Jake Delhomme and Seneca Wallace one quarter each on Saturday. ... CB Brandon McDonald practiced after missing two days with a tender ankle. ... RT John St. Clair was carted off after cramping up during post-practice sprints in the swampy heat.